1. Field of the Invention
The present invention relates to a fuel cell usable as a power source for an electronic device or the like.
2. Description of the Related Art
Currently, secondary batteries, such as lithium ion batteries, are mainly used as power sources for electronic devices, such as portable notebook personal computers (hereinafter referred to as notebook PC's), mobile devices, etc. These modern electronic devices have increasingly higher functions and require increased power consumption and longer operating time. To meet these requirements, compact, high-output fuel cells that require no charging are expected as novel power sources. Various types of fuel cell exist. A direct methanol fuel cell (hereinafter referred to as DMF) that uses a methanol solution as its fuel, in particular, has advantages over one that uses hydrogen as its fuel; easier fuel handling and simpler construction. Thus, the DMF is a power source for an electronic device that is currently drawing a lot of attention.
Normally, a DMF has a case that houses a fuel tank, mixing tank, liquid pump, air pump, etc. The fuel tank contains high-concentration methanol. The methanol in the fuel tank is diluted with water in the mixing tank. The liquid pump pressure-feeds the methanol that is diluted in the mixing tank to an electromotive unit. The air pump is used to supply air to the electromotive unit. The electromotive unit has an anode and a cathode. It generates power based on chemical reaction by feeding the diluted methanol and air to the anode and cathode sides, respectively. As this is done, the electromotive unit is heated to high temperature by reaction heat that is produced by a chemical change. In general, the amount of heat produced by a fuel cell is proportional to the amount of power generated by it.
According to a fuel cell described in Jan. Pat. Appl. KHAKI Publication No. 7-6777, for example, heat that is produced by power generation is discharged into the case via the surface of the electromotive unit and anode and cathode passages. Air within the case is discharged for ventilation with a cooling fan or air blower that is attached to the inner surface of the case. Thus, the fuel cell can be kept at a desired operating temperature without undergoing an excessive increase in temperature.
As reaction products that result from the power generation in the fuel cell described above, carbon dioxide and water are produced on the anode and cathode sides, respectively. As mentioned before, the heat produced by the power generation is discharged through the anode and cathode passages. However, some of the water as a reaction product is discharged in the form of steam into the case of the fuel cell. If the fuel cell is cooled by the steam discharge through the cathode passage, therefore, the water is gradually converted into steam and reduced, so that necessary water for the power generating reaction cannot be secured satisfactorily. In consequence, the power generating capacity of the fuel cell inevitably lowers despite the presence of the residual fuel.